Here are Thursday's coldest pre-dawn temperatures in San Diego County

HPWREN/UCSD

A cold air mass moving through San Diego County.

A cold air mass moving through San Diego County. (HPWREN/UCSD)

Gary Robbins

The tail of a cold air mass dropped temperatures into the 20s, 30s and 40s before dawn on Thursday. The daytime high will only reach the mid-50s in San Diego, and the 40s in most other parts of the county.